Moraxella catarrhalis, also known as Moraxella (Branhamella) catarrhalis or Branhamella catarrhalis and formerly known as Neisseria catarrhalis or Micrococcus catarrhalis, is a gram-negative bacterium frequently found in the respiratory tract of humans. M. catarrhalis, originally thought to be a harmless commensal organism, is now recognized as an important pathogen in upper and lower respiratory tract infections in animals. In humans, M. catarrhalis causes serious lower respiratory tract infections in adults with chronic lung disease, systemic infections in immunocompromised patients, and otitis media and sinusitis in infants and children. See Helminen et al., 1993, Infect. Immun. 61:2003-2010; Catlin, B. W., 1990, Clin. Microbiol. Rev. 3:293 320; and references cited therein.